Prestressed concrete beam



P- 9, 1969 o. ZALDASTANI ET AL 3,465,484

PRESTRESSED CONCRETE BEAM Original Filed Feb. 20. 1967 INVENTORS OTHARZALDASTANI MICHAB. J.A.-H. JOLLIFFE XF V ATTOR N EYS United StatesPatent 3,465,484 PRESTRESSED CONCRETE BEAM Othar Zaldastani and MichaelJ. A. H. Jolliife, both Nichols, Norton, and Zaldastani, Inc., 131Clarendon St., Boston, Mass. 02116 Continuation of application Ser. No.617,181, Feb. 20, 1967. This application Oct. 22, 1968, Ser. No. 772,466Int. Cl. 1304c 3/26; EtMb 5/04 US. Cl. 52-127 3 Claims ABSTRACT OF THEDISCLOSURE This application is a continuation of Ser. No. 617,181 filedFeb. 20, 1967 and now abandoned.

BACKGROUND OF THE INVENTION The field to which the invention pertainscomprises precast reinforced concrete beams intended for support of orintegration with a variety of deck materials including concrete, whetherprecast or poured in place, lumber and metallic deck systems.

The prior art includes two well-known precast structural systems usingreinforced concrete load-bearing members, namely, slab units andintegral beam-and-deck units. In any such structure the load inducestensile stresses in the lower portions and compressive stresses in theupper portions of load-bearing members. Since concrete is relativelyweak in tension, the tensile stresses are borne almost entirely bylongitudinal reinforcing wires around which the concrete is cast.

In any practical slab structure, inefliciency in the use of concretetends to increase greatly with an increase in the span and load. Much ofthe concrete in the portions under tensile loading has little utility.The integral beamand-deck units overcome this limitation to a degree,but have other defects. They are relatively costly to produce and lackflexibility in installation and use under different loadings within abuilding. Also, they are often too costly where deck materials otherthan concrete could be used. In many instances the structures rest upontransverse girders which limit the overhead clearance.

Some work has been done on precast, prestressed concrete beams, that is,beams cast around reinforcing wires that are held deflected and undertension in the mold until after the concrete has set. After severance ofsuch wires from the mold and removal of the beams therefrom, the beamshave a slight curvature inverse to that of the intended load, suchcurvature being termed the camber. Undesirable variations occur in thecamber of beams previously in use, largely due to differences inthickness of concrete at different parts of the cross section. Also,lateral structural instability exists in some cases, necessitating crossbracing for longer spans.

Other disadvantages exist in one or more of the foregoing structures,including difliculty in storing the precast units and in erecting themon the site, and also in making satisfactory and stable connections tosupporting members.

3,465,484 Patented Sept. 9, 1969 ice SUMMARY OF THE INVENTION Thisinvention avoids the foregoing disadvantages by providing an easilyprecast, prestressed beam of novel configuration and design, and onethat is readily removed from the mold. In cross section the beam may beconsidered as comprising a combination of three elements, namely, twovertically tapered and oriented, parallel reinforced web portions, and avertically tapered transverse and continuous, reinforced concrete flangeportion or diaphragm connecting and integrally joined with the webportions. These three portions act compositely to form the beam unit.Such beams may also act compositely with other structural or deckelements which they support, depending on the nature of such elementsand the mode of connection or support. The diaphragm or flange provideslateral stability to the beam. Also, it may be extended longitudinallyso as to act as an end bracket for support by other structural elementssuch as girders. Further, the composite beam defines a space that may beusefully employed for acoustical absorption or for lighting fixtures,pipe and ductwork of any desired description.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary longitudinalelevation in section showing a projecting end of a preferred embodimentof the beam.

FIG. 2 is a cross section of the beam showing details of the endreinforcing bracket.

FIG. 3 is an elevation in section of a midportion of the beamillustrating its use in conjunction with telescoping forms for casting adeck, and also showing the use of perforated panels closing the spacedefined by the beam and thus forming a resonator which absorbs sound bythe Helmholtz effect.

FIG. 4 is an elevation in section of an end of the beam and the decksupported thereby, both resting upon a notched concrete girder.

FIG. 5 is a plan view corresponding to FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 shows a typical crosssection near the middle of the beam, and FIG. 2 shows a cross sectionnear one end. The beam is designated generally at 12 and comprises anintegrally cast, reinforced structure including two web portions 14 anda flange or diaphragm portion 16. The beam is cast in a mold of theappropriate length for a span to be supported, for example from 40 feetto feet. The side walls of the mold are preferably tapered so as toproduce a beam having outer sides 18 sloping inwardly toward the bottom,and inner sides 20 having the reverse slope preferably with the sameangle to the vertical as the sides 18. The taper of the sides 18 and 20is preferably about one part in sixteen measured in relation to thevertical.

The beam preferably has continuous, longitudinal, tapered notches 22 ofuniform cross section extending along its uppermost shoulders or edges,such notches being adapted for support of retractable or telescopingforms 24, such forms being placed on the beams after the latter havebeen erected in the structure to act as temporary supports for casting acontinuous deck 26 over the beams.

The distance s between central vertical lines of symmetry through theweb portions 14, shown in FIG. 2, is preferably sixteen inches, which isa modular building dimension. The thickness of the web portions at theirlowest extremities is preferably about five inches which is sufficientto allow room for two spaced vertical rows of reinforcing wires. Forsimplicity of illustration, however, the described embodiment employs asingle row of reinforcing wires 28 in each web, such rows being locatedwithin the web portions 14 where tensile stresses exist under loadconditions, and along the aforementioned lines of symmetry. It will beunderstood that in manufacture, the wires 28 are correctly locatedwithin the casting form and held under tension between the ends of theform, ordinarily with a point or points between the ends restrained inpositions closer to the bottom than the points of restraint at the ends.This is in accord with conventional construction methods employed in themanufacture of prestressed concrete beams.

End reinforcing brackets designated generally at 30 are also placed inthe ends of the mold form. These brackets may be formed in various ways,the preferred form for the illustrated single row of wires comprising apair of steel plates 32 each welded to a plate 33, a pair of relativelythick rods 34 welded to each plate 32 in such manner as to embrace therow of longtiudinal wires 28, wire extensions 35 of the plates 32, and anetwork of wires or stirrups 36 each of U-shape, welded to the plates 32and wires 35. Also, U-shaped stirrups 37 are ac curately located inspaced relationship along the entire length of the form between the endreinforcing brackets.

With the foregoing reinforcements in place, the concrete is cast intothe form with the stirrups 36 and 37 projecting above the surface of theconcrete, and allowed to harden for about 16 to 24 hours with theprestressing wires 28 held in tension as stated above, during which timethe reinforcing wires and brackets become firmly bonded to the concreteand after which the beam may be stripped from the mold. This merelyrequires cutting the ends of the wires 28. When the beam has beenremoved from the mold it has a measurable upward bow in the center,referred to as camber. It has been found that this camber is veryuniform for beams of the same dimension, and this fact is extremelyhelpful both in storing the beams and in erecting them on the site. Theuniformity of camber is attributable largely to the illustrated crosssection configuration which comprises the two web portions 14 and theconnecting flange portion 16.

A graded family of beam sections may be produced in the foregoing mannerfor different loads and spans. Preferably, the modular distance s is thesame in all members, but the inside vertical clearance depth d and theexternal overall depth D are variable, the former preferably from about12 to about 36 inches, and the latter from about 18 to about 48 inchesin two-inch intervals, thereby accommodating any span from about 40 toabout 120 feet with typical deck construction and loading, as in aparking garage. Throughout this family of sizes the taper of the sides'18 and 20 of the webs is also preferably a constant. Thus for deeperwebs than those illustrated in the drawing the flange is of greaterlateral width as well as greater depth. The dimensions of the flangeportion are a vital structural consideration, since this portion bearsthe compressive load on the beam and provides its lateral stability.

The foregoing configuration comprises a very efiicient section, namelyone having a large area for distribution of compressive stress balancingthe tensile stress on the wires 28. Also, the proportion of the variousdimensions discussed above is such as to reduce the possibility ofcracks forming in the beam, including those caused by shrinkage as wellas by loading.

Beams formed in the manner described above are brought to the site anderected between walls or girders 38 (FIGS. 4 and 5), the beams beingspaced on 6- to 15- foot centers. To this end each beam preferably hasan endwise projection formed by an extension 16a of the flange portion16, the projection resting within a notch 40 in the girder. To preventstress concentration the beam has a chamfer 42 at the juncture of theextension 16a with the ends of the web portions 14. This chamber opposesa corresponding chamfer on the lip of the notch 40. A clearance space44, preferably of about inch, is provided between all surfaces of thebeam and the girder, the beam resting on a neoprene pad 46 ofapproximately the same thickness as the desired clearance space. Thisspace is to accommodate dimensional tolerances and movements of the typenormally encountered in a building structure, such as those attributedto temperature changes and plastic flow.

As shown in FIG. 4, a deck 26 is cast upon the top surface of the beamover the stirrups 36 and 37, and extends to the end of the extension16a. The deck may be cast in place using the retractable forms 24 aspreviously described; or a pre-formed deck may be applied in the form ofsheets or planks, either of concrete or of any other suitable deckmaterial.

One variation of the structure may be accomplished by eliminating theprojecting stirrups 36 and 37, in which case the resulting structuredoes not become a composite one in which the deck 26 assumes a part ofthe compressive load with the flange portion 16 as previously described.

As previously stated, the cross section of the beam provides alongitudinal space, designated as 48, which may have any one or more ofseveral useful functions. Inserts 50 may be cast into the beam forattachment of hanger wires 52, at spaced intervals, such wiressupporting means such as Z-brackets 54 in which perforated soundabsorption panels 56 may be suspended. The panels 56 thus enclose thespace 48 as an acoustical sound absorption space. In this case, soundabsorption takes all of the forms described in Patent No. 2,933,146issued Apr. 19, 1960, to Zaldastani and Junger, namely, the Helmholtzresonator effect, the black body effect and an organ pipe effect.

The tapered inner walls 20 contribute to the black body effect bycausing dispersion of sound energy by reflection within the space 48.

The Helmholtz effect is expressed by the relationship between the volumeof the chamber, the total area of openings 58 provided in the panels 56and the thickness of the panels, in accordance with the formula given insaid patent. These values may be varied in relation to one another totune for suppression of any frequency between about 250 and 1,000 cyclesper second.

The organ pipe effect is a function of the depth d of the space 48,which is one-fourth of the wavelength of the frequency suppressed.

Also, sound absorptive material can be placed inside the cavity formedby the perforated panels and the beam, and thus provide additional soundabsorptive properties.

It will be obvious that the space 48 may also be employed for enclosingelectrical wiring, conduits, ductwork, pipes and the like. This may bedone without sacrifice to the head space within the structure, or itsappearance. At the same time, all such enclosed objects remain easilyaccessible for maintenance or other purposes.

It will be apparent that in place of notches 40 as shown in FIG. 5, thegirders 38 may have continuous ledges of the cross section shown in FIG.4. Also, it is possible to eliminate the end extensions 16a of the beam,resting its web portions 14 either upon the top surfaces of thesupporting members such as walls or girders, or upon ledges or innotches as described above. Other structural variations may be employedconsistently with the features of this invention, and without departingfrom its spirit or scope.

We claim:

1. An integrated structural deck system having, in combination,

a pair of spaced supports,

a plurality of cast, generally parallel, mutually spaced, prestressedconcrete beams each supported at its ends by said supports and having apair of spaced, parallel, longitudinal web portions of substantiallyuniform trapezoidal cross section and height, uniformly tapered sidewalls and decreasing width toward the bottom, said web portionscontaining wire prestressing strands, and a flange portion joining theweb portions and forming a smooth continuation of the noncontiguous sideWalls thereof, said flange portion having a depth sufiicient todistribute the compressive stresses corresponding to the tensilestresses in the web portions under load, the height of the beams beingat least the distance between the center lines of their web portions,the beams being spaced on centers a distance greater than their height,and a deck extending over the beams and the spaces be tween the beamsand being supported thereby.- 2. A deck system according to claim 1, inwhich said flange portion has continuous notched shoulders defining anupper surface, said shoulders being adapted for support of retractableforms, and said flange portion also has metal reinforcing memberspartially imbedded in its upper surface between said shoulders, and thedeck is cast upon the top surfaces of the forms and beams and over saidreinforcing members, the deck providing clearance for subsequentretraction of the forms.

3. A deck system according to claim 2 in which a beam has suspensionmeans within the space defined by the Web and flange portions, andperforated panel means supported by said suspension means in position toenclose said space to define a tuned resonant sound-absorption chambertherein.

References Cited UNITED STATES PATENTS 1,891,763 12/1932 Henderson 52602X 2,007,374 7/1935 Kuehne 52145 2,130,285 9/1938 Marqua 52283 X2,428,304 9/1947 Abeles et al. 52-723 X FOREIGN PATENTS 675,943 12/ 1963Canada. 1,166,294 6/1958 France.

821,409 11/1951 Germany.

5/1944 Great Britain. 7/1964 Switzerland.

ALFRED C. PERHAM, Primary Examiner US. Cl. X.R.

